So just to get things straight:
Jun was summoned from Japan by the Goddess to defeat the 1st demon king, and was blessed with the powers to both defeat the king and purify its soul.
Because she could not physically fight, Jun bestowed the power to defeat the king on her knight, Marcus. The power to purify still stayed with her; the theory was that Marcus would bring back the demon king's soul for her to properly finish the job.
The reigning king, the head priestess' ancestor, feared Jun's powers would threaten his authority, so he framed her as a demon and left her to die at the hands of a mob.
Marcus presumably absorbs the demon king's soul in revenge, thus making him the new demon king.
Because the job to end the demon king is still unfinished, the Goddess blesses new heroes with the defeating power, while Jun's soul is perpetually reincarnated with the purifying power. Her memories, and progressive incarnations, have been sealed to prevent trauma.
Previous heroes have only ever defeated Marcus's shadows, and in the end Marcus eventually kills them and absorbs their powers; all while seeking for his beloved's soul.
Various earthly conspiracies and plots have sought to use Louisa's previous incarnations' connection to the hero of the time, or to remove her to get closer to the hero. Nearly all of them ended in their deaths.
Marcus's absorbed powers will cause the apocalypse of the world if he absorbs Graham.